Adjustable band-pass selector



Y- 1941- J. F. FARRINGTON ADJUSTABLE BAND-PASS SELECTOR Filed Feb. 12, 1936 2 Sheets-Sheet 1 4 3 mog E8. an 5 58:0 5.

INVENTOR. F. FARRING N ATTORNEY.

.Nov. 11, 1941. J. F. FARRINGTON ADJUSTABLE BAND-PASS SELECTOR Filed Feb. 12, 1936 2 Sheets-Sheet 2 QUON 0 INVENTOR. N F. FARRIN ON ATTORNEY.

Patented Nov. 11, 1941 ADJUSTABLE BAND-PASS SELECTOR John F. Farrington, Flushing, .N. Y., i

Hazeltine Corporation, a corporation of Delaware Application February 12, 1936, Serial ideas 11' Claims, (01.250-401 Y This invention relates to carrier-frequency signaling systems and particularly to band-pass selectors for such systems adjustable to control the selectivity thereof.

In radio broadcasting systems'in general use, programs are transmitted from each station on a carrier frequency which ordinarily has two sidebands of signal-modulation frequencies ex,-

tending 6 kilocycles or more on each side of the I carrier frequency. The various broadcastingstations are allotted carrier frequencies which are uniformly spacedv throughout the frequency range, usually 10 kilocycles apart. In many instances, therefore, the sideband frequencies of one carrier overlap those of adjacent carriers received at the same location, or closely en'- croach thereon. This condition frequently renders it difi'lcultto tune abroad-cast receiver to a desired signal'without interference from undesiredsignals on' carrier frequencies'near the desired signal carrier, particularly when the strength of the undesired signal is comparable to, or exceeds that of, the desired signal. Static and other so-called background noises,'which are ordinarily at'the higher frequenciesof the sidebands, also frequently interfere with quiet operation. Various types ofselecting systemshave been devised for use in such receivers to pass a band of desired modulation frequencies su-fliciently narrow to reduce greatly the undesired-signals and noise. Such narrowing of the band, however, tends to impair the fidelity of reception of the desired signals, since the outer frequencies of the sideb'ands, corresponding in radio broadcasting to the higher audio'frequencies'of modulation, are suppressed. Therefore, it is'highly desirable that selecting systems bemad'e adjustable so that the width of the selected band may be contracted when an undesired signal or noise -of sufficient amplitude to cause interference is Heretofore, the adjustment of the width of the band of frequenciespassed by aselector system has been accomplished, in most instances, by varying" the mutual reactance between reactively coupled circuits or'by' detuning circuits of the system relative to each other. Certain systems have also been'devised whereby the adjustment of the bandwidth is accomplished by the use of adjustable resistorsl systems of the latter type, however, whilehaving certain advantages, have not proved entirely satisfactory forvarious reasons, chief among-which being that, in order to obtain the desired'band-pass characteristic in such systems, a plurality of cascaded-- selecting systemsor relatively complicated arrangements have beennecessartm 1 r It is an object ofthe present invention to provide an improved carrier frequency band-pass selector system-by means. of which there may be procured optimum'fidelity of transmission consistent with the intensities of .the desired and undesired signals near thedesired signal carrier frequency l l, I

i More particularlygit is an object of the .ine vention to provide :a-=band-pass selectorsystem of the type described, wherein adjustable resistive impedance means are employed in a simple and effective arrangement for effecting the selectivitycontrol. 1 M i v It is a further object-of the invention to pro.- videa selector system of the type described, wherein vacuum tube conductances are utilized as theadjustable impedance means. i

'A further object is to provide: a selector system of the character described, whereby the selectivity is automatically controlled in accordance with the amplitudes of the received signals.

For a better understanding of myinvention, together with other and further objects thereof, reference is had to. the following description taken in connection-with the accompanying drawings;- and its scope .will be pointed out in the appended claims. i

In accordance with the present invention, an electric circuit arrangement for controlling the selectivity of a modulated-carrier signal bandpass selector comprises input and output terminal circuits'and an intermediate circuit reactively coupling said terminal circuits. Each-of these circuits is a resonant circuit and is tuned to the desired carrier frequency, the coupling between the circuits being, in the preferred embodiment of the invention, by mutual inductance.

The coupling is initially adjusted in accordance with well-known design principles to obtain the maximum desired transmission frequency band.

of the band of frequencies passedby the selector.

Adjustable resistive impedance means, such as a vacuum tube conductance, is also preferably effectively connected in circuit-with-one ofthe terminal circuits, to provide aproperterminaf tion impedance for the selector and effect uniform transmission of frequencies in the band passed by the system. "By adjusting the impedance means in the intermediate circuit'to'in crease the decrement thereof and thus, in efiect, to decrease the coupling of the'input and output circuits and by simultaneously adjusting the im pedance means in the terminal circuit to reduce the losses therein, successivelynarrower transmis-- sion characteristics are obtained. Smooth transi tion from maximum to minimum band width is effected by the provision of unicontrol means for simultaneously adjusting the impedance means in both of the circuits.

In accordance with a preferred embodiment of the invention, the width ofjthe band of fre quencies passed by the selectonis automatically controlled by adjusting the adjustableimpedance means in accordance with the amplitude of desired signal input to the selector; Means are also provided responsive to the amplitude of an illi-j desired signal on a carrier frequency near the desired signal carrier frequency to vary the amplitude of the desiredisignal input to the selector inversely with the amplitude of the undesired signal. v Thus, the adjustment of the'band width of the selector is automatically varied inversely in accordance with the amplitude of the undesired signal. This arrangement preferably includes also an amplifier preceding the selecting system the amplification of which is automatically'controlled in accordance with the amplitude of the selected carrier to maintain the signal output thereof within'a relatively narrow range for a wide range of signal input amplitudes. and the same control means is utilized to eflect both adjustment, of the ban-d width and the automatic amplification control. I I, I

Also in accordance with the invention, a radioirequency receiving system comprises a plurality of cascade-coupled amplifiers and includes means for controlling the selectivity thereof in accordance with the relative amplitude of potentials derived respectively from electrical energy in a limited range of frequencies and from electrical energy in frequency bands adjacent to the limited range, The controlling means comprises at least one coupling device for at least two of. the amplifiers; thecoupling device including tuned circuits connected with each of the amplifiers and a link circuit for coupling the tuned circuits. The system includes also means responsive to the relative amplitude of the above-mentioned potentials for varying the effectiveness of the link circuit in coupling' 'the tuned circuits.

In the accompanying drawings, Fig. 1 is a circuit diagram of a selector system embodying the present invention; Fig. 2is a group of curves representing certain operating characteristics of the selector system of the invention, to aid in the understanding thereof; Figs, 3 and 4 are circuit diagrams of other forms of the invention; Fig. 5 is a circuit diagram of a complete super heterodyne receiver, partially schematic, embodying the present invention; and Fig. 5a is a curve illustrating theresonance characteristic of the broad band intermediate-frequency amplifier of the receiver shown in Fig. 5.

Referring now more particularly to Fig. 1, there is represented an electric circuit arrangement for controlling the selectivity of a modulated-carrier signal band-pass selector embodying the present invention in which the transmission band is directly controlled by adjustable resistors. A source of carrier-frequency voltage is represented by the amplifier tube ID having means II for applying signal voltage to its control grid. The tube I0 may be, for example, an intermediate-frequency tube of a superheterodyne receiver. Connected to the output'circuit of the tube II] is the selector system of the present invention, comprising a plurality of resonant circuits including, an input circuit I, an output circuit III and an intermediate coupling, or link, circuit II.

Eachof these circuits comprises one or more inductance elements I2 and one or more tuning condensers l 3, by means of which the circuits are individually tuned to a selected frequency, for example, the intermediate-carrier frequency, where the system is used in the intermediatefrequency portionof a superheterodyne receiver. Inductance elements l2 of the circuits I and II and of the circuits II and III, respectively, are inductively coupled. The couplings between the circuits I and'lI and between II and III are initially adjusted in accordance with well-known design principles to obtain the -maximum desired transmission frequency band.

A resistive impedance means, for example, a resistor 14, is connected eifectively-in circuit with the intermediate circuit II, either in series as shown, or, if desired, an equivalent shunt resistor may be utilized. Resistive impedance means is also preferably connected effectively in circuit with one of the terminal circuits I or This means may comprise -a shunt resistor 15, connected acrossthe circuit III as shown, or an equivalent series element, and may be included in either circuit III or I.

With the selector circuit elements initially adjusted to provide maximum band width transmission, upon adjustment of the resistor l4 grad-- provide a proper termination resistance 'for the 1 selector and a fiat-top or uniform transmission characteristic therefor. It is initially adjusted to meet these requirements when the system is adjusted to provide the maximum desired band width. By simultaneously adjusting the resistor M to increase the losses in the circuit II, and resistor IE to decrease the losses in the circuit 111 so that a proper termination impedance for the selector is maintained for all band-width adjustments, the desired transition from broad band to sharp selectivity characteristic may be obtained. The adjustable elements of resistors l4 and I5 are preferably connected by unicontrol means, as indicated at U, for the simultaneous control thereof, to effect a smooth transition from maximum to minimum band width.

It is to be understood that it is purely a matter of expediency as to whether series or shunt resister are employed as the control elements. Where shunt resistors are employed they should be connected across the entire inductance of the circuit, since otherwise detuning would occur as the resistance is varied. The L/C ratios in the tuned circuits may be chosen with regard to available control elements, or vice versa. Furthermore, it is not necessary that all tuning inductances and capacitances be alike, and this freedom of choice facilitates the adaptation of the circuit to available control elements.

It is further pointed out that, while mutual inductive coupling between the several circuits is shown in the embodiments of the invention herein illustrated, other forms of coupling may be used, as for example, direct inductive or capacitive coupling. For best performance and in order to preserve the symmetry in the transmission characteristic of the selector, it is important that the coupling between circuits I and III shall be negligible except for that which is provided through circuit II. For this reason, where mutual inductive coupling is used, two pairs of coupled coils, as shown in Fig. l, is a preferred arrangement.

In Fig. 2, a group of frequency-gain curves is shown illustrating the band expression characteristics obtainable with systems constructed in accordance with the present invention. The abscissae in these curves represent frequency displacement in kilocycles from the mid-band frequency to which the circuitsof the system are tuned, which is indicated as 0, while the ordinates represent in decibels the ratio of the output voltages at the mid-band frequency and at the indicated sideband frequencies. The curves A, B, C, D, E, F and G illustrate respectively successively narrowed transmission frequency bands resulting from changes in the decrement or power factor of the intermediate circuit and one of the terminal circuits, as for example, the circuits II and III in Fig. 1. To change from broad band to narrow band conditions, the losses in circuit II were varied between minimum and maximum limits while the losses in circuit III were simultaneously varied in opposite sense, although not necessarily between the same limits as those of circuit II.

In changing the transmission frequency band conditions between maximum and minimum limits the transmission through the selector may not be equally eflicient as measured at the mid-band frequency. It is possible to proportion the losses introduced for band-width control purposes so that the mid-band-gain-band-width characteristic can be controlled within reasonable limits to give level, rising, or falling gain characteristics.

The adjustable band-pass selectors of this invention are particularly advantageous in that they provide adjustable band-width characteristics having substantially flat-top transmission within desired adjustable frequency limits and high and uniform attenuation on the skirts of the curves outside the fiat-top band. Such characteristics result from proper proportioning of the fixed tuned circuit constants and from proper proportioning of the losses in the termination and intermediate circuits.

impedance elements In the particularembodiment of the invention that was used to obtain the characteristic curves of Fig. 2, the selector was designed for a mid-band frequency of 175 kc. and transmission between desired maximum frequency limits of i7 kc. The tuned circuit constants were approximately as follows: All

coils-4.8 millihenries, all condensers-170 micro- 'microfarads' and mutual inductance between the paired coils-.56 m'illihenry. Since, in an arrangement such as is shown in Fig. 1,

where the inductances La and Lb are the total inductances of the respective coupled'circuits in the present instance times, that of this coeificient of coupling, and this provides the desired range of selectivity adjust,- ment. 1 Y

In Fig. 3 there is illustrated an embodiment of the present invention in which vacuum tube conductances are utilized to effect the band-width adjustment instead of resistors such as are employed in the embodiment shown in Fig. 1. Here, as in the embodiment of Fig. 1, the selector comprises three reactively coupled resonant circuits I, II and III connected to the output circuit of a vacuum tube amplifier l0. Vacuum tubes I6 and I! are employed as a the adjustable resistive in circuits II and 'III, respectively, their space-current paths being effectively in circuit with their respective resonant circuits. The anode-cathode circuit of the tube 16 is connected in shunt with the maximum impedance point of the circuit II, that is, across the whole inductance of this circuit, and the anodecathode circuit of the tube I1 is connected in shunt with the maixrnum impedance point of the circuit III. The tubes I6 and I! are shown'as vider comprising resistors I8, I9 and 20 and are connected to ground through high-frequency bypassing condensers 2| and 22, respectively.

A source of unidirectional voltage for controlling the selectivity of the system is applied to terminals 23 of a voltage divider 24. Resistors 25 and 26, connected in series and grounded at their junction; are connected across an adjustable portion of thevoltage divider 24. in this case equal, variable voltages from "the terminals of resistors 25 and 26 are applied to the grids of the tubes [6 and II, respectively. In the grid lead of each of the tubes- 5 and H is included a series resistor 21 and a shunt condenser 28, constituting filter networks for suppressing alternating voltages on the control grids. In the output of the circuit III a coupling condenser 29 and shunt resistorv 30 are also'provided, constituting a filter for passing the selected band of frequencies to the succeeding part of the Opposite, and

' ously described arrangements. tion resistance is provided by the dynamic plate reduced to zero; that the cathode bias of.;the t bs i6 is such that the plate impedance ofthis :tube is' low, so that the losses and power ,factor of the circuit II are high a nd.that the cathode.

bias on the tube fl gis suchthat this tubehas-a very high plate impedance, so that the losses of the circuit III are ata These conditions correspond to minimum transmission band width, that is, greatest, selectivity of the system.

Thereafter as the; control-voltages applied to the grids'of, the tubes [6 and I! are increased in opiposite senses, the impedance of the tube It increases, thus loweringjthe losses and power factor of circuit II, while the impedance of the tube I1 decreases, thus increasing the losses and power factor of circuit III, until maximum transmission 7 band-width conditions are attained, at which time the plate impedance'of thetube l6 is'very .high, approaching infinity, andthe plate impedance of the tube l'l corresponds to the correct' shunt termination resistance for the maximum-handmassconditions.

Selectivity curves similar to those of Fig. '2 were obtained from tests of an embodiment of the invention shown in Fig. 3, with control bias adjustments, in volts, as follows:

Curve 7 Tube '16 [Tube 17 B est +5.5 +5

' V The were type pentodes with 180 volts applied to their anodes and cathode biases of 5.5 volts and 14 volts, respectively.

- by the system. l

, As the alternating-current impedance of the described constitutes, in general, a conventional Referring to Fig. 4, there is shown a simplified g-s form of the'inventio'n also utilizing vacuum tubes to control the damping of the circuits and there- "by effect the band-width adjustments. In this arrangement the adjustable termination impedance means is applied in shunt with circuit I, instead of with circuit III, as in the two previ- This terminaimpedance of the pentode vacuum tube 3!, which also acts as a signal amplifier. A tube 32 is connected in shunt with the circuit H, substantially in the :same manner as'the tube 16 in the emt onpo ent leeraslua w ut-pfi tube-fo xamp e a type 8;.-tube; pr ferablyemhlqyed i hispos By ad u t en c t v a e v d th d m i g of t ir s'l a d I ma be e cted simultaneously and in opposite: senses, thereby .to

adjust the width of the frequency band passed tube 3|. is reduced by increasingthe negative potential applied to its suppressor grid, the mutual conductance of the tube also diminishes, thus reducing the gain of this tube as a signal amplifier.

Suchperfo-rmance, however, is generally desirable in cases where the selectivity is controlled automatically since the change in gain brought aboutin this manner may cooperate with orbe used in lieu of conventional automatic amplification control.

Referring -now to- Fig. 5, there complete superheterodyne receiver embodying the presentinvention in a manner whereby adjustment of the selectivity of the receiver is effected automatically in accordance with the amplitudes ofboth the desired and the undesired signals atthe input of the receiver. In general, the receiver includes a tunable radio-frequency amplifier and frequency changer 4;] having its input connected with an antenna 4| and ground42 and its output connected to an intermediatefrequency amplifier .43 by means of a band-pass selector system 44. A selective coupling network 45 serves to couple the output of the intermediate-frequency amplifier 43 to a rectifier 46, which is in turn coupled to an 41.

rectifier 46 and the automatic amplificationcon- .trol means form-parts of, or cooperate with,-the

system of the present invention and will be hereinafter described in more detail.

Neglecting for the moment the operation of these parts of the receiver, the apparatus just superheterodyne receiver. The operation of such a receiver is well-known in the art and a detailedexplanation thereof is, therefore, unnecessary. In brief, however, signals intercepted by the antenna'are selected, amplified and converted to intermediate frequencies in the well-known manner in the tunable radio-frequency amplifier and frequency changer 49. The intermediate-frequency signals are transmitted by the selector system I 44 to the intermediate-frequency amplifier 43 bodiment shown 'in'Fig. 3, for controlling the I damping of this circuit. Tube 32 is shown as a pentodeconnected and operated as a 'triode although, 'of course, any other suitable type of tube maybe employed. A source of unidirectional voltage, indicated at 33, is appliedacross a voltage divider 34 one end of which is grounded and the adjustable tap of whichis connected to the "control grid of the tube 32' and the suppressor grid'of' tube 3|, through'series resistors 35 and shunt condensers 36, which serve as filters substantially in the same manner as described with reference to the correspondingelements in Fig. 3.,

The plate impedance of the tube 3| is regulated I by the adjustment'of a negative biasing potential 'applied'to its suppressor grid. The impedance of the tube ismaximum when the bias is zero and diminishesto'low values with increasingnegative 75,

wherein they are further selected and amplified and from which they are delivered to the recti- -fier 46, whereby the audio-frequency signals are derived. 1 The audio-frequency signals are transmitted to the audio-frequency system 47 wherein these signals are amplified andsupplied in the ;-usual manner-to a loudspeaker for reproduction.

- The selectorsystem 44in this embodiment of the invention is'similarin many respects to the embodiment shown in Fig. 3, and only its distinguishing features, therefore, will be described.

A single tuning condenser 13 is employed for the circuit II but is equivalent to the two corresponding condensers of Fig. 3-. Control vacuum tubes 48 and 49, corresponding to the tubes l6 and IT in Fig. 3, are here shown as triodes, although the same type of tubes as used in the other embodiment may, of course, also be employed here. The tubes 48 and 49 are connected and operate substantially in the same manner as the control tubes of Fig. 3, excepting that herethe control is illustrated a audio-frequency system The selector system 44, coupli ng network 45,

. fier, 12.

electrodes of the respective tubes are connected, for automatic control, with the output circuit of the rectifier 46 as will presently be described.

The selective coupling network 45 is a broad band coupling and may comprise an intermediate-frequency transformer including a primary winding 50, which is tuned to the intermediate frequency by a condenser and is coupled to the output of the last stage of the intermediatefrequency amplifier 43, and a secondary winding 52, which is tuned to the intermediate frequency by condenser 53 and comprises the input circuit of the rectifier 46. The windings 50 and 52 are coupled sufliciently to provide the desired broad band transmission characteristic when the winding 52 is properly terminated.

The rectifier 46 is a diode having connected between its cathode and anode a load circuit which includes series-connected resistors v54, 55, 56 and 51, the resistors 54 and 55 being shunted by a high-frequency by-pass condenser 58 and the resistors 56 and 5'! being shunted by a similar condenser 59. The resistors 55 and 56 are also shunted individually by high-frequency by-pass condensers 60 and 6|, respectively. The common junction between the condensers 58-59 and 60-6I and the resistors 55-56 is grounded. The unidirectional voltage developed across the resistor 55 is applied negatively through the lead 62 by way of a large time constant filter including series resistor 63 and shunt condenser 64 to the control electrode of one or more of the tubes of the intermediate-frequency amplifier 43 and to the control grid of the tube 48 of the selector system 44. A further filter including series resistor 65 and shunt condenser 66 is also included in the lead 62 to the grid of tube 48. The unidirectional voltage simultaneously developed across the resistor 56 is app-lied positively through the lead 61 by way of a large time constant circuit including a series resistor 68 and shunt condenser 69 to the control grid of the tube 49. The audio-frequency voltage developed by the load circuit is transmitted by means of connection to the audio-frequency system, as shown.

The control of the selectivity is accomplished in this embodiment of the invention automatically in accordance with the amplitude of both the desired and undesired signal input to the receiver. The amplification in the intermediatefrequency amplifier 43 is controlled in accordance with the amplitude of the desired signal input to the rectifier 46 by means of the biasing voltage developed across resistor 55 and applied to the grids of one or more of the tubes of the intermediate-frequency amplifier as above described. The selectivity of the selector system 44 is also controlled by the biasing voltage developed across resistors 55 and 56 which are proportional to the desired signal input amplitud to the amplifier 43.

To procure selectivity control of the selector 44 by the undesired signals, an auxiliary broad band intermediate-frequency amplifier H is connected to the output of the radio-frequency amplifier and frequency changer 40, and the output of the amplifier H is, in turn, connected to an automatic amplification control, or first AVC recti- The radio-frequency amplifier 40 is designed to pass a band of frequencies which is at least as wide as the fully expanded band of the selector system 44, this relation being essential to obtain the full benefit of expansion. The intermediate-frequency amplifier ll, however, is designed to pass a band of frequencies which is sub stantially wider than that of the radio-frequency amplifier; that is, it passes and amplifies not only the desired signal but also the undesired signals which are passed by the radio-frequency amplifier and frequency changer 40 and which have sufficient amplitude to be capable of causing interference or overloading the frequency changer.

The rectifier I2 is designed and operated in conventional manner to develop a unidirectional biasing voltage proportional to the amplitude of the signal input thereto. The biasing voltage thus developed is applied negatively to the control grids of one or more of the tubes of the radio-frequency amplifier and frequency changer 40 to control the amplification therein inversely in accordance with the amplitude of the input to the rectifier 12.

Hence, in the presence of interfering signals, the desired carrier input amplitude to the intermediate-frequency amplifier 43 is reduced, with the resultant contraction of the band by the selctor 44, as previously explained.

Since, as before mentioned, the undesired signals which are most likely to cause interference are thos on carriers adjacent the desired signal carrier and separated therefrom by about. 10 kilocycles in the standard American broadcast system, the intermediate-frequency amplifier 1| in the preferred embodiment of the invention is designed to have a characteristic such as is illustrated by the curve of Fig. 5a. In that figure relative gain in decibels is plotted against frequency difference in kilocycles from the intermediate frequency indicated as O. Thus, the amplifier II is designed to be more responsive to adjacent carrier frequencies than to the desired carrier frequencies or other frequencies. Hence, by virtue of this characteristic, the adjacent undesired signals have relatively a substantially increased effect; that is, a relatively greater contraction of the band width is effected bythese signals than by desired signals or other signals. The construction and operation of amplifiers having a characteristic curve such as that shown in Fig. 5a is well understood by those skilled in the art and a detailed description thereof is deemed unnecessary.

The rectifier 12 together with the rectifier 46 also provide double automatic volum control, the control by rectifier 12 serving to keep the signal level well below the overload level in the first part of the receiver and the control provided by the rectifier 46 serving, as above described, to maintain the amplitude of the output of the amplifier 43 substantially uniform. By thus avoiding overloading of the receiver, distortion of the desired signal'and cross-modulatron of the desired signal by undesired signals will be obviated.

The amplification of the intermediate-frequency amplifier 43 is controlled not only by the application of the biasing voltage applied to the control electrodes thereof from. the rectifier-46, as above mentioned, but may also be controlled to a certain extent, by action of the selector 44. I

Reviewing briefiy, then, the general operation of the receiver shown in Fig. 5, it is noted that an mcreasein the bias voltage developed by the rectifier 46 has the effect of controlling both the amplification of the intermediate-frequency amplifier 43 and the selectivity of the selector system 44. The amplitude of the desired signal carrier input to the intermediate-frequency amplifier 43 is the factor which directly determines the degree of amplification therein and the band width of the selected. band.- In the absence of undesired signals this; general relation is'suffi? cient for satisfactory reproduction andlno other type of control is needed. When, howeventhere is present an undesired signal on-a frequency near; the: desiredsignal carrier frequency and of suflicient amplitude to be capable of causing interference, this signal is passed. by the broad bandintermediate-frequency amplifier H to the rectifier 1'2 and the control biasing voltage developed by this rectifier is utilized to control the radio-frequency amplifier and thereby effect a reduction of the desired, signal. input amplitude to, the intermediate-frequency amplifier 43. Since the. rectifier 46- is responsive to the :output of the amplifier 43' regardless of whether the amplitude thereof has been varied in response to. desired or undesired signals, the undesired signals. will thus indirectly effect. a contraction of'the frequency band passed lbylthe selector; In summary,..therefore, the. circuitarrangement of Fig- 5.provides, in. aradio receivingsystem,,. a, plurality. of. cascade-coupled; amplifiers 4.0. and 43. and including. means for controlling the. selectivity. thereofv in. accordance with the relative amplitude of the potential de'rivediby the rectifier 46. from electrical; energy in a limited range of frequenciesand. passed; by selectors43 and 45; from the potential derived by. rectifier -12 from electrical energy infrequency bands, adjacentto the. limited range passed by amplifier 1].. The selectivity-controlling means. comprises at least one coupling device for .the two amplifiers. and..the coupling device includesrtunedcincuits I. and; III. connected with the amplifiers respectively and. a link. circuit .II for coupling thewtunedcircuits, I and\.III.. The circuit of Fig.

5 additionally includes means responsive to Y the coupling the. tuned circuits I and-III='.

, While I have described whatl at presenttconsider the, preferred embodimentsitof my invention,i,t wi1l be obvious to thoseskilledduthe art that, Various, changes andmodifications may be made, therein withoutzdeparting from my invention and I, therefore, aim in, the appended claims to, cover, all; such changes and modifications. as fall: within the, true: spirit; and. scopes-of. my in- Yn i9n. I i What isclaimed isz, v 1.. An electric circuit-arrangement. for :control-' ling the; selectivity. of.,-a.modulated-carrier signal band-pass selector comprising-input and, output terminal circuits and an intermediate circuit I67 actively couplingsaid terminal circuits, eachof said circuits being a resonant .circuittuned. to the desired carrier frequency, resistive ='impedance means effectively in circuit with said..intermediate circuit I and adjustable, ,over., ia, i eluding. a minimum value proyidingraypower factor for said circuit which is:;substantially..le ss than the coefiicientjof couplingbetween. said.in-.- termediate circuit and each of j said terminal circuits anda maximum value providingapower factor for said intermediate circuit iWl'iiChLiS' .sub-

stantially greater than the said; coefficient; of coupling, and means for adjusting;said.imped-.

ance' means over said rang e, thereby substan-' circuit. with said intermediate circuit andi ad- I resonantcircuit tuned. to a selected; carrierifre 'quency, resistive impedance means" effectively in circuit which is substantially greater than: the

said coeflicient of coupling, and means for. ad.- justing: said impedance means over said range; and thereby substantially adjust the width. of the frequency band passedby said:system..- V

3. A. carrier-frequencyband-pass selectorsystem comprising input and output terminal cir cuits and an intermediate circuit coupling said terminal circuits, each, of said circuits being: a resonant circuit tuned to a selectedcarrier frequency, resistive impedance means effectively in circuit with said intermediate, circuit, resistive impedance means effectively in circuit With one of said terminal circuits, means for simultaneously adjusting both said impedance meansto vary. the damping of their associated circuits in opposite senses andthereby'adjust the width of the frequency band passed by said system,.the relative adjustments of said impedance means being proportioned to maintain substantially the same. rate of. variation of attenuation-with re'-, spect to frequency at frequencies-; outsideof the selected band, for all said. band-width. adjust-v ments.

4. In a carrier-frequency. signaling-system in= eluding a vacuumtube amplifier, a band-pass selector comprising input: and: output. terminal circuits and an .intermediate circuit coupling, said terminal circuits, each of said: circuits-being. a resonant circuit: tuned to a selected. carrier irequency and said inputcircuit being coupled.- to the'output of .said amplifier for receiving signals therefrom, the space-current path of. said: vacuu-mtube amplifier. serving as .an'; adjustable 5 re: sistive impedariceain said input circuit, adjustable resistive impedance means coupled'to said intere mediatecircuit, andmeans: for controlling. the

conductanceof said :tube. andtsaid; impedance means simultaneously. to adjustxthe damping of theirassociated circuits in opposite senses, there. by: to adjust the width of; the bandof frequencies passed' by saidsystem.

- 5."An electric. circuituarrangementl for: control:- lingthe-selectivity of. amodulated-carrier signal band-pass selector to. discriminate against; an undesired-signal"on a. carrierufrequency'r near the desired v carrier frequency: comprising three resonant circuits individually tuned to. the desired carrierfrequency and including input;and:output circuits and an intermediate circuit," said circuits being reactively coupled torpass: albandofifretquencies between desired frequencies: ofsmaxie mum separation, resistive-impedance means effectively! in circuit: with said intermediate cit.- cuitand adjustable, over a., range: including a minimum value providing ;a powerrfactor forisaid intermediate: circuit. which issubstantiallydess than. the; coefficient of couplingbetweensaid 1 intermediate; circuit and: each, of; said terminal circuits and a, maximum'valueproviding av power factor: for. said: intermediate vcircuit which isot the 'orderofitemtimes the value, of said coefiir cientof coupling, .and; means: for adjusting, said impedance meansxover said; range; thereby; substantially to: adjust then-width; of: the frequency. bandgpassedhy said'selecton i ,p

6. A carrier-frequency band-pass selector system comprising three resonant circuits individually tuned to a selector carrier frequency and including input and output circuits and an intermediate circuit, said circuits being reactively coupled to pass a band of frequencies between desired frequencies of maximum separation, resistive impedance means effectively in circuit with said intermediate circuit and adjustable over a range including a minimum value providing a power factor for said intermediate circuit which is substantially less than the coefficient of coupling between said intermediate circuit and each of said terminal circuits and a maximum value providing a power factor for said intermediate circuit which is of the order of ten times the value of said coefiicient of coupling, and means for adjusting said impedance means over said range, thereby substantially to adjust the width of the frequency band passed by said system.

'7. A carrier-frequency band-pass selector system comprising three resonant circuits individually tuned to a selected carrier frequency, including input and output terminal circuits and an intermediate circuit, said terminal circuits being reactively coupled to said intermediate circuit to a predetermined extent and said intermediate circuit and one of said terminal circuits normally having power factors of substantially zero, whereby said circuits will pass a band of frequencies between desired frequencies of maximum separation, adjustable resistive impedance means effectively in circuit with said intermediate circuit and normally having negligible effect on the power factor thereof, and means for adjusting said resistive impedance means to increase the power factor of said intermediate circuit and thereby decrease the width of the band of frequencies passed by said system.

8. A carrier-frequency band-pass selector system comprising input and output terminal circuits and an intermediate circuit coupling said terminal circuits, each of said circuits being a resonant circuit tuned to a selected carrier frequency, said intermediate circuit and one of said terminal circuits normally having power factors of substantially zero, whereby said circuits will pass a band of frequencies between desired frequencies of maximum separation, adjustable resistive impedance means effectively in circuit with said intermediate circuit and normally having negligible effect on the power factor thereof, adjustable resistive impedance means effectively in circuit with said other of the terminal circuits and normally adjusted to provide a predetermined maximum power factor therefor to effect a proper termination impedance for said selector, and

means for simultaneously adjusting said impedance means to increase the power factor of said intermediate circuit and thereby decrease the width of the frequency band passed by said system and to decrease the power factor of said other termination circuit to provide a proper termination impedance for said selector at all band-width adjustments.

9. A radio receiving system having a plurality of cascade-coupled amplifiers and including means for controlling the selectivity thereof in accordance with the relative amplitude of potentials derived respectively from electrical energy in a limited range of frequencies and from electrical energy in frequency bands adjacent to said limited range, in which said means comprise at least one coupling device for at least two of said amplifiers, said coupling device including tuned circuits connected with each of said amplifiers and a link circuit for coupling said tuned circuits, and means responsive to the relative amplitude of said potentials for varying the effectiveness of said link circuit in coupling said tuned circuits.

10. In a modulated-carrier signal receiver, a band-pass selector, for selecting a desired signal comprising a carrier and a band of modulation frequencies, including input and output terminal circuits and a link circuit reactively coupling said terminal circuits, and means responsive to the amplitude of an undesired signal on a carrierfrequency near the desired signal-carrier frequency for varying the effectiveness of said link circuit in coupling said terminal circuits to adjust the width of the band of frequencies passed by said selector inversely in accordance with said undesired signal amplitude.

11. In a modulated-carrier signal receiver. a band-pass selector, for selecting a desired signal comprising a carrier frequency and a band of modulation frequencies, including input and output terminal circuits and a link circuit reactively coupling said terminal circuits, means for varying the effectiveness of said link circuit in coupling said terminal circuits to adjust the width of the band of frequencies passed by said selector, means responsive to the amplitude of said desired signal for controlling said adjusting means automatically to adjust said band width directly in accordance with the amplitude of said desired signal, and means responsive to the amplitude of an undesired signal on a carrier frequency near the desired signal carrier frequency for. automatically controlling said adjusting means to adjust said band width inversely in accordance JOHN F. FARRIANGVTON. 

